Customarily, these current feedthroughs are made of niobium (see German Patent Specification 14 71 379). However, they are only partly suitable for lamps that are intended for a long useful life. This is due to the strong corrosion of both the niobium tube and the ceramic material used for sealing the niobium tube into the plug when the lamp has a metal halide fill. An improvement is described in the U.S. Pat. No. 4,545,799, Rhodes et al. The niobium tube is tightly sealed into the plug by the shrinking process of the "green" ceramic during the final sintering without ceramic sealing material. This is readily possible because both materials have approximately the same coefficient of expansion (8.times.10.sup.-6 K.sup.-1). Reference to the contents of this specification is expressly made.
Current feedthroughs made from other metals have also been tested. The U.S. Pat. No. 4,011,480, Jacobs et al, and 4,160,930, Driessen et al, describe discharge lamps in which tile tubular current feedthroughs consist of tungsten, molybdenum or rhenium. The tube is supported by a ceramic cylinder which has straight, axially aligned walls and is disposed in the interior of tile tubular current feedthrough. The cylinder may be either solid or hollow; in the latter case, the bore serves as the exhaust tube and is closed later. The seal between the feedthrough and tile ceramic parts engaging the feedthrough on the inside and on the outside thereof, which have been finally sintered at a temperature of 1850.degree. C. prior to the finishing of the seal, is still made by using a ceramic sealing material. Although this improves the corrosion resistance of these lamps, it still does not satisfy the requirements as desired for metal halide fills. In spite of great efforts, it has not been possible heretofore to develop a ceramic sealing material that is capable of resisting corrosion.